Collins won her last reelection bid in 2014 by more than 30 points but is expected to face a much tighter race this time around, with the leader of the state's House of Representatives, Sara Gideon (D), announcing she would challenge the four-term senator.

The senator defended her vote to confirm Kavanaugh last month in an interview with The New York Times, telling the newspaper she did not regret her vote "in the least."

Gideon, meanwhile, hammered Collins for the vote in her campaign announcement earlier this year.

“At one point, maybe Sen. Collins was different, but she doesn’t seem that way anymore: taking over a million dollars from drug companies and the insurance industry and voting to put Brett Kavanaugh on the Supreme Court,” Gideon said.

Collins's campaign did not immediately return a request for comment from The Hill.

A spokesman for the National Republican Senatorial Campaign (NRSC) told The Hill in a statement that Gideon was "plagued by scandal," and characterized Gideon's primary opponent Betsy Sweet as "too liberal."

“Susan Collins has the most bipartisan record in the U.S. Senate because she works with members of both parties to deliver results for Maine families. She is in a strong position because voters know that Collins is an independent voice for Maine, while Sara Gideon is plagued with scandal and Betsy Sweet is too liberal," Nathan Brand said.

Little polling exists of the 2020 Maine Senate race so far, but a Gravis poll taken in June indicated that Collins had a 14-point lead over Gideon.